1. Field of the Invention
The present invention relates to the field of ceramic materials. More specifically, this invention relates to a dense silicon carbide material having a density close to theoretical and an extremely high electrical resistivity making it especially suitable for use in plasma etching chamber applications.
2. Background Art
SiC ceramics are exceptional structural materials with high hardness, chemical and mechanical wear resistance, low thermal expansion coefficient, high thermal conductivity and good mechanical properties. However, SiC is a semiconductor and in most cases its electrical resistivity is lower than about 1×107 Ω·cm, preventing its use as an electrical insulator, e.g., a substrate for integrated circuits or an electrostatic chuck or as other components in dielectric etching chambers. In addition to the desirable properties listed above, for most semiconductor etching chamber applications, the purity of the material especially with respect to all metallic impurities has to be as high as possible. This is difficult to achieve in most cases where α-SiC is required as a starting powder, as it is made using the Acheson process and typically not very pure.
High resistivity SiC materials would have a broad applicability in many areas where electrical insulation is required. The advantage of SiC compared to other ceramics is that it has good thermal conductivity, low thermal expansion coefficient and good mechanical strength. For applications in Si wafer etching chambers, in addition to these properties, the material also needs to have very low etching rates in corrosive atmospheres containing F, Cl and other etching gas combinations that form plasmas and electromagnetic fields during operation. The material also needs to have very high purity towards most metals so that they do not contaminate the etched wafer surface during the process. In addition, the material should not have free carbon as a separate phase in the microstructure, as this causes particle generation on the wafer surfaces.
It is well known that Be containing compounds are very powerful sintering aids for SiC densification, and that sintering or hot pressing with their additions also provides a high resistivity SiC material by controlling the n- and p-type charge carrier populations in the material (over 1×109 Ω·cm, page 145, Silicon Carbide Ceramics-2, ed. S. Somiya, Elzvier 1991). Unfortunately, Be containing powders which are required for the production of these materials can cause Chronic Beryllium Disease (CBD) and are therefore not commercially available. In addition, such materials contain Be in amounts not desirable in etching chamber applications.
Another approach to electrically insulating SiC materials is by providing an electrically insulating oxide based grain boundary layer between the SiC grains. U.S. Pat. No. 6,531,423 patent (Schwetz) is one example where a Yttrium Aluminum garnet phase at triple points and Y—Al—Si—O glassy grain boundary phase surrounding SiC grains provides a material with electrical resistivity of >1×107 Ω·cm. Canadian Patent No. 1,256,459 teaches a similar material with additions of Al2O3 or MgO, in addition to free carbon, yielding resistivities of over 1×109 Ω·cm. Although such materials have a high resistivity, the Al, Mg and Y (and other common sintering aids) additive levels in them are too high.
A third group of insulating SiC materials are based on having a Boron (elemental or as a compound) as one of the additives, which typically provide a directly bonded SiC material, without a distinct grain boundary phase.
U.S. Pat. No. 4,701,427 teaches a dense, high resistivity SiC obtained by pressureless sintering of formed bodies in a nitrogen containing atmosphere (at over 2250° C.) of predominantly α-SiC powders. Additions to the SiC powder include 0.4-2.0% B4C and free carbon (additions of which are required up to 2.5%) either as carbon or as a temporary binder which will yield amorphous carbon on decomposition.
Japanese Patent Application No. Japanese Patent Application No. 2002-078977 (Publication No. 2003-277152) (Sumitomo Metal) teaches a dense SiC material with 0.5-4% free carbon (from organic precursors) and 1-20% BN additions that is hot pressed and has a resistivity of over 1×108 Ω·cm.
Japanese Patent Application No. 2005-057149 (Publication No. 2006-240909) (Sumitomo Osaka) teaches a dense SiC material made by pressureless sintering in a inert atmosphere of SiC powders with additions of 0.5-8% free carbon, 0.1-4 wt % boron additions (as boron or its compounds) and 0.1-4.0 wt % Si3N4 additions. The material has resistivities above 1×106 Ω·cm.
Japanese Patent Application No. 2003-358764 (Publication No. 2005-119925) similarly teaches a dense SiC pressureless sintered material made by adding 0.5-10% free carbon and 0.2-4 wt % B4C or BN powders to the starting SiC powder. This SiC material is disclosed to have resistivities higher than 1×107 Ω·cm if it has 650-8000 ppm of nitrogen content.
Japanese Patent Application No. 2005-057149 (Publication No. 2006-240960) (Toshiba Ceramics) teaches high resistivity SiC ceramic (>1×108 Ω·cm) made by pressureless sintering of α-SiC powder with additions of 0.7-1.0 wt % BN powder and 0.5-4 wt % free carbon (or organic precursor resulting in latter). The resulting material has to have 0.4-0.5% nitrogen content to obtain claimed high resistivity.
Japanese Patent Application Nos. 1986-0006394 (Publication No. 62167253) and 1985-0290454 (Publication No. 62153167), both Showa Denko KK, disclose a SiC material with resistivity, respectively >105 Ω·cm or >1×104 Ω·cm, both of which contain 0.5-3.0 wt % free carbon in addition to B, Al and less than 300 ppm of nitrogen in the densified ceramic.
All of the materials disclosed above have in common that additions of free carbon (either as a powder or an organic precursor) had to be made to the starting SiC powder. Although the addition of carbon helps the densification, the residual free carbon, present as inclusions of varying sizes in the finished part, is detrimental to the performance: it impairs the insulating property and mechanical behavior of the sintered part; it is believed that it also degrades the erosion/corrosion resistance when used in plasma environment and leads to increased contamination. Therefore, there is a need to develop processing and composition that does not use carbon as additive and contains no residual free carbon in the finished parts.
Japanese Patent Application No. 2001-352223 (Publication No. 2003-146758) (Sumitomo Cement) teaches a SiC sintered material that contains less than 100 ppm of free carbon and less than 1000 ppm of all elements (measured by ICP) other than Si and C. The material is made by adding 1-10% BN powder to the SiC powder, hot pressing the material to about 90% of theoretical density, followed by surface machining and a heat treatment in Argon that is claimed to purify the material. Open porosity present in the material presumably allows the purification step to be effective. This material is porous and is not suitable for many applications due to this, including sustaining vacuum and uneven wear in etching chambers.
U.S. Pat. No. 3,960,577 describes a hot pressed β-phase SiC material made with 0.3-3.0% B-addition and 3.5-10% Si3N4 additions. The material is described as having nitrogen and B atoms accommodated in the SiC lattice which result in a conductive SiC material (resistivity of only 50 Ω·cm). Japanese Patent Application No. 11-210817 (Publication No. 2001-130971) (Bridgestone Corp.) application teaches a low electrical resistivity SiC (˜1000 Ω·cm) material sintered using Si3N4 and free carbon precursor additions to the SiC starting powder (>65 wt % β-SiC). The resistivities in these inventions are very low and would not solve the problem this invention solves. U.S. Pat. Nos. 5,011,639 and 4,762,810 both describe a process for obtaining high resistivity SiC by pressureless sintering fine β-SiC powder obtained by pyrolysis of methyl hydrogen disilane, to which boron compounds such as B4C, titanium boride and B2O3 are added, pressed and processed in a furnace with a source of nitrogen in the atmosphere. Japanese Patent Application No. 1986-61-100421 (Publication No. 62-256764) (Shin-Etsu) teaches a method for obtaining high resistivity SiC made by adding B2O3, BN or B and other metal oxides to β-SiC powder and pressureless sintering it on BN plates to densities over 3.09 g/cm3. All patents mentioned in this paragraph involve β-SiC materials and differ substantially from the present invention because of this. In addition to this difference they all involve different additives as compared to the present invention.
U.S. Pat. No. 4,135,938 (Carborundum) teaches a SiC with a resistivity of only ˜1-10 Ω·cm made by sintering of α-SiC powders with the additions of BN, BP or AlB2 (in the ˜1 percent range) with simultaneous additions of 1-4% free carbon. Sintering was done between 2100 and 2200° C. in Ar. The findings in this invention are substantially different from the findings in patents described earlier with similar starting compositions, and the material has a low resistivity, which is not desirable.